1. Field of the Invention
The present invention relates to an integrated coal gasification combined cycle plant for combined power generation using coal as fuel, and in particular, to an increase in the plant efficiency in a flue-gas-desulfurization-type integrated coal gasification combined cycle plant.
This application is based on Japanese Patent Application No. 2004-188886, the content of which is incorporated herein by reference.
2. Description of Related Art
Heretofore, a flue-gas-desulfurization-type integrated coal gasification combined cycle (hereinafter referred to as “IGCC”) plant has been known. In such a flue-gas-desulfurization-type IGCC plant, an exhaust gas in a gas turbine contains a large amount of a sulfur component. However, since a heat recovery steam generator (HRSG) provided at the downstream side of the gas turbine includes a carbon steel heat exchanger therein, sulfuric acid produced from the sulfur component causes a corrosion problem.
In order to prevent this corrosion, the gas temperature at the outlet of the heat recovery steam generator must be set at a value (for example, about 150° C.) higher than the dew point (130° C.) of sulfuric acid. However, when the gas temperature at the outlet of the heat recovery steam generator is set at a high value, thermal energy of a gas recovered from the exhaust gas of the gas turbine at the heat recovery steam generator is decreased. Consequently, the output of a steam turbine which utilizes steam generated by heat recovery steam generator is decreased, thereby decreasing the plant efficiency.
According to a proposed example measure for the above-described corrosion caused by sulfuric acid, by adjusting the amount of recirculation in a deaerator, the gas temperature at the outlet of the, heat recovery steam generator is controlled to a value equal to or higher than the dew point of sulfuric acid. In this method, the corrosion can be prevented by increasing the temperature of a metal at the heat transfer surface which is in contact with the gas, but the gas temperature also increases at the same time. Consequently, the amount of heat recovery is decreased, resulting in a problem of reduced power generation efficiency (see, for example, Japanese Unexamined Patent Application, Publication No. Hei-11-148603).
In the above flue-gas-desulfurization-type IGCC plant, the plant efficiency is desirably increased by recovering the maximum amount of thermal energy from the exhaust gas (combustion exhaust gas) of the gas turbine. Therefore, preferably, by increasing the thermal energy recovered from the combustion exhaust gas of the gas turbine, even when the gas temperature at the outlet of the heat recovery steam generator is lower than the dew point of sulfuric acid, the corrosion problem of the heat recovery steam generator can be solved, thus realizing an IGCC plant with high efficiency.